Nowadays, the major energy source for humankind comes from petroleum. The power or electricity required to drive a car or run a thermal power plant is supplied by burning petroleum. However, the heat and exhaust generated during the combustion of the petroleum not only can deteriorate the air quality, but can worsen the global warming effect. Besides, the yield of the petroleum will reach culmination in ten years and then will decline year by year. This means that the oil price (including the electricity tariff) will not be cheap anymore. Therefore, the energy crisis might come up eventually and cause global economic storm.
In view of the forthcoming global economic storm, renewable energy has been discovered to provide electricity or mechanical power efficiently and economically for households or industries. Thus far, the development of renewable energy has become an important energy policy for developed countries as a win-win strategy for power generation and environmental protection. Among various renewable energy, such as solar energy, wind energy, tidal energy, geothermal energy, and biowaste energy, the solar energy has become the mainstream as the solar energy generator has the advantages of high eco-friendliness, easiness of installation, matureness of commercial merchandising, and the overwhelming promotion lead by the country. Hence, solar energy has become a major choice for developed countries in pursuing distributed power supply system.
Referring to FIG. 1, in which the circuitry of a DC-AC converter according to the prior art is shown. As shown in FIG. 1, the DC-AC converter 1 is used in solar grid-connected photovoltaic system, and thus the DC-AC converter 1 is also known as a photovoltaic inverter, or PV inverter. The DC-AC converter 1 is configured in an non-isolated and full-bridge topology, and includes an input filter 10, a full-bridge switch circuit 11, and an output filter 12. The input filter 10 is consisted of a first capacitor C1 that receives a DC input voltage VDC generated by a solar cell and filters the DC input voltage VDC. The full-bridge switch circuit 11 is connected to the output filter 12 and is consisted of switch elements S1-S4, in which the first switch element S1 is connected in series with the second switch element S2 and the third switch element S3 is connected in series with the fourth switch element S4 so as to form a full-bridge circuit with two bridge arms. The switch elements S1-S4 are controlled by a control unit (not shown) to turn on or off, thereby allowing the full-bridge switch circuit 11 to convert the filtered DC input voltage VDC into an AC modulating voltage VT. The output filter 12 is connected to the full-bridge switch circuit 11 and is consisted of a first inductor L1, a second inductor L2, and a second filtering capacitor C2. The output filter 12 is used to remove the high-frequency components of the AC modulating voltage VT to output an AC output voltage Vo to a grid G.
Generally, the switch elements S1-S4 of the full-bridge switch circuit 11 are configured to operate under the pulse-width modulation (PWM) fashion. More precisely, the switch elements S1-S4 of the full-bridge switch circuit 11 can operate under the bipolar switching mode or the unipolar switching mode depending on the operation mode of the switch elements S1-S4. Referring to FIGS. 2 and 3, in which FIG. 2 is the waveform diagram of the modulating voltage of the full-bridge switch circuit of FIG. 1 which is operating under the bipolar switching mode, and FIG. 3 is the waveform diagram of the modulating voltage of the full-bridge switch circuit of FIG. 1 which is operating under the unipolar switching mode. As shown in FIGS. 1-3, the switch elements S1 to S4 are configured to operate with a high frequency under the bipolar switching mode. Under this condition, the AC modulating voltage VT outputted by the full-bridge switch circuit 11 is fluctuated between the positive DC input voltage VDC and the negative DC input voltage −VDC during the positive half-cycles or negative half-cycles, as shown in FIG. 2. Under the unipolar switching mode, only one bridge arm of the switch circuit is operating with a high frequency while the other bridge arm of the switch circuit is turned off during each half-cycle. That is, the bridge arm consisted of the first switch element S1 and the second switch element S2 and the other bridge arm consisted of the third switch element S3 and the fourth switch element S4 are turned on and off alternately, thereby allowing the AC modulating voltage VT outputted by the full-bridge switch circuit 11 to fluctuate between 0 and the positive DC input voltage VDC during the positive half-cycles and fluctuate between 0 and the negative DC voltage −VDC during the negative half-cycles, as shown in FIG. 3.
As the full-bridge switch circuit 11 is operating under the unipolar switching mode, only one bridge arm consisted of two switch elements are configured to conduct high-frequency switching operations, instead of allowing two bridge arms consisted of switch elements S1-S4 to conduct high-frequency switching operations under the bipolar switching mode, the AC modulating voltage VT is fluctuating between 0 and the positive DC input voltage VDC or fluctuating between 0 and the negative DC input voltage −VDC. Therefore, the switching loss of the full-bridge switch circuit 11 operating under the unipolar switching mode is less than the switching loss of the full-bridge switch circuit 11 operating under the bipolar switching mode. In other words, the full-bridge switch circuit 11 will have better conversion efficiency under the unipolar switching mode. However, as a parasite capacitance Cp is existed between the solar cell which generates the DC input voltage VDC and the ground terminal, as shown in FIG. 1, the modulating voltage VT will have high-frequency components when the full-bridge switch circuit 11 is operating under the unipolar switching mode. Thus, the relative voltage drop between the first output terminal A′ and any node within the DC-AC converter 1, such as the relative voltage drop between first output terminal A′ and the common mode N′ connecting the parasite capacitance Cp, and the relative voltage drop between the second output terminal B′ and the common mode N′, can not be set to maintain their total average value at any switching point at a constant value. This would result in a significant voltage drop across the parasite capacitance Cp and cause leak current, thereby endangering human users and equipment. If the full-bridge switch circuit 11 is operating the bipolar switching mode, the leak current can be avoided.
Thus, the applicants endeavor to develop a DC-AC converter with a better conversion efficiency and lower leak current.